Addressing a matrix-type liquid crystal cell

ABSTRACT

A liquid crystal cell comprising a matrix of pixels defined by areas of overlap between members of first and second sets of orthogonal electrodes has its pixels selectively set to first and second states by addressing the pixels in rows. The pixels a given row are first set no a first state by applying an erase signal (12) to the corresponding row conductor simultaneously with applying bipolar data signals (8, 9) to all the column conductors. Selected pixels of the row are subsequently set to a second state by applying a strobe signal (10) to the row conductor simultaneously with applying a bipolar data signal(9) to the relevant column conductors, another bipolar data signal (8) being applied to the column conductors corresponding to non-selected pixels. The start of the strobe signal coincides with the start of the active central portion of the data signals but the strobe signal continues beyond the end of this portion, enabling the addressing rate to be increased. (FIG. 2)

This invention relates to a method of addressing a matrix-type liquidcrystal cell including liquid crystal material which is electricallysettable to first and second stable optical states, the cell comprisinga plurality of pixels which are defined by areas of overlap betweenmembers of a first set of electrodes on one side of the material andmembers of a second set of electrodes, which cross the first set, on theother side of the material, in which method the pixels are addressed inlines, the addressing of each line comprising (a) applying an erasesignal having a given polarity to the corresponding electrode of thefirst set while applying at least one charge-balanced bipolar datasignal to each electrode of the second set, thereby setting any pixel ofthe line which is not already in the first state to that state, and (b)subsequently applying a strobe signal having an opposite polarity tosaid given polarity to the corresponding electrode of the first setwhile applying a charge-balanced bipolar data signal to each electrodeof the second set, thereby selectively setting to the second state anypixel of the line for which the data signal applied to the correspondingelectrode of the second set has a given form. The invention also relatesto optical modulator apparatus for implementing such a method.

A known such method is disclosed in GB-A-214473. In this known method,the data signal comprises first and second successive portions ofopposite polarities. When the data signal has the given form the strobesignal coincides with that one of these portions which has the givenpolarity, so that the magnitude of the signal applied across thecorresponding pixel is the sum of the magnitudes of the strobe and datasignals. GB-A-214473also discloses an addressing method in which thedata signal comprises first, second and third portions, the polarity ofthe second portion being opposite to that of the first and thirdportions. In this last method the strobe signal coincides with thesecond portion of the data signal and is bipolar, making it unnecessaryto employ an erase signal to ensure that all pixels of a line areinitially in the first state. In both these known methods the product ofthe time for which the data signal has one polarity and its amplitudewhen it has this polarity is equal to the product of the time for whichit has the other polarity and its amplitude when it has this otherpolarity. The data signal is therefore balanced; in itself it has no neteffect on any given pixel.

The maximum addressing speed when using such methods is limited by thelength of the erase signal (if present) and the length of the strobesignal; each has to be present for a sufficient time to ensure that therelevant pixels are actually set to the first and second statesrespectively, this process taking a finite time. When the first of theknown methods referred to above is used, the length of the strobe signalis equal to the length of one of the two (equal-length) portions of thedata signal i.e. to half the length of the complete data signal, andwhen the second of these known methods is used the length of the activeportion of the strobe signal is equal to half the length of the secondportion of the data signal, which corresponds to one quarter of thelength of the complete data signal.

It is an object of the invention to allow the minimum length of acomplete data signal in a method as defined in the opening paragraph tobe reduced, thereby increasing the maximum permissible addressing speed.

According to one aspect the invention provides a method as defined inthe first paragraph which is characterised in that when said data signalhas said given form it comprises first, second and third successiveportions in which it has said given polarity, said opposite polarity andsaid given polarity respectively, the amplitude of the second portionbeing less than the amplitude of the strobe signal, the end of the firstportion coinciding with or occurring before the start of the strobesignal, the end of the second portion occurring after the start of thestrobe signal, and the start of the third portion occurring before theend of the strobe signal.

It has been found that, if the data signal of the given form has first,second and third successive portions which are respectively of anopposite polarity to, the same polarity as, and an opposite polarity tothe strobe signal, and the amplitude of the second portion is less thanthat of the strobe signal, then it can be beneficial in respect of thesetting to the second state if the strobe signal continues into theperiod occupied by the third portion of the data signal. Theco-operation of the strobe signal with at least part of the thirdportion of the data signal can assist the setting effect of theco-operation of the strobe signal with at least part of the secondportion of the data signal, thereby enabling the time for which it isnecessary that the strobe signal co-operates with the second portion ofthe data signal, and hence the minimum actual duration of the secondportion of the data signal, to be reduced.

According to another aspect the invention provides optical modulatorapparatus comprising a matrix-type liquid crystal cell and an addressingsignal generator for addressing said cell, the cell including liquidcrystal material which is electrically settable to first and secondstable optical states, and comprising a plurality of pixels which aredefined by areas of overlap between members of a first set of electrodeson one side of the material and members of a second set of electrodes,which cross the first set, on the other side of the material, theaddressing signal generator having outputs coupled to the first andsecond sets of electrodes and being contructed to address the pixels inlines by each time (a) applying an erase signal having a given polarityto the corresponding electrode of the first set while applying at leastone charge-balanced bipolar data signal to each electrode of the secondset, thereby setting any pixel of the line which is not already in thefirst state to that state, and (b) subsequently applying a strobe signalhaving an opposite polarity to said given polarity to the correspondingelectrode of the first set while applying a charge-balanced bipolar datasignal to each electrode of the second set, thereby selectively settingto the second state any pixel of the line for which the data signalapplied to the corresponding electrode of the second set has a givenform, characterised in that the generator is constructed so that whensaid data signal has said given form it comprises first, second andthird successive portions in which it has said given polarity, saidopposite polarity and said given polarity respectively, the amplitude ofthe second portion being less than the amplitude of the strobe signal,the end of the first portion coinciding with or occurring before thestart of the strobe signal, the end of the second portion occurringafter the start of the strobe signal, and the start of the third portionoccurring before the end of the strobe signal.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings in which

FIG. 1 shows optical modulator apparatus comprising a liquid crystalcell and an address signal generator therefor;

FIG. 2 shows various signals occurring in the apparatus of FIG. 1, and

FIG. 3 illustrates how the states of pixels in the cell of FIG. 1 areset by means of the signals depicted in FIG. 2.

In FIG. 1 a matrix-type liquid crystal cell 1 comprises in known mannera pair of transparent plates which are superimposed one upon the otherwith a small spacing therebetween which contains ferroelectric liquidcrystal material. The cell comprises a plurality of picture elements(pixels) which are defined by areas 2 of overlap between members of afirst set of parallel transparent electrodes 4 provided on the innersurface of one plate, i.e. on one side of the liquid crystal material,and members of a second set of parallel transparent electrodes 3provided on the inner surface of the other plate, i.e. on the other sideof the liquid crystal material. The electrodes 3 and the electrodes 4cross each other and in the present example are oriented substantiallyorthogonal to each other and each corresponds to a respective line ofpixels. (With the orientation shown each electrode 3 corresponds to arespective column of pixels and each electrode 4 corresponds to arespective row).

The cell 1 is addressed by means of an addressing signal generator 5 viaconductors which are connected to respective electrodes 3 and conductors7 which are connected to respective electrodes 4. For each pixel theresulting electric field applied there across determines the alignmentof the liquid crystal molecules and hence the optical state of thatpixel. The cell 1 is positioned between parallel or crossed polarizers(not shown). The orientation of the polarizers relative to the alignmentof the liquid crystal molecules determines whether or not light can passthrough a pixel in a given state. Accordingly, for a given orientationof the polarizers, each pixel has a first and a second opticallydistinguishable state provided by the two stable states of the liquidcrystal molecules in that pixel.

The signals produced by generator 5 are shown in FIG. 2. Generator 5 isconstructed to generate a succession of bipolar data signalssimultaneously on each column conductor 6, these data signals beingsynchronised with each other and each occupying a time 2T. These datasignals each take one of two forms, denoted by reference numerals 8 and9 respectively in FIG. 2. During each data period 2T generator 5 alsogenerates a signal on each row conductor 7, these signals each takingone of three forms, denoted by reference numerals 10,11 and 12respectively in FIG. 2. The result is that the pixels 2 are addressedwith data in rows (although addressing in columns could equally well beemployed, if desired).

If a given row is considered, then the complete addressing of the pixels2 therein includes the steps of (a) applying the signal 12 to thecorresponding row conductor 7 during at least one data period 2T whileapplying one of the data signals 8 and 9 to each of the columnconductors and (b) applying the signal 10 to the corresponding rowconductor 7 during a subsequent data period 2T while applying one of thedata signals 8 and 9 to each of the column conductors. Which of the datasignals 8 and 9 is applied to which of the conductors in step (b)determines the final state of each of the pixels 2 in the relevant row,as will now be explained.

FIG. 3 of the drawings is a graph illustrating conditions necessary toobtain switching of a given pixel from one of its opticallydistinguishable states to the other. In this Figure the unidirectionalvoltage V applied across the pixel is plotted along the axis ofabscissae and the time t for which this voltage is applied is plottedalong the axis of ordinates. It is assumed that the polarity of thevoltage V is such as to tend to switch the pixel from its current stateto the other state. The graph shows a curve 13 having the general formof a "U". Points within the "U", i.e. within the region 14, correspondto actual switching of the state of the pixel, whereas points outsidethe "U", i.e. within the region 15, correspond to the pixel remaining inits current state. (It should be noted that the exact shape and positionof the "U" depend upon various factors, such as temperature, the voltageconditions existing immediately prior to the time period t underconsideration, etc). Examples of values for V_(d), (V_(b) -V_(d)),(V_(b) +V_(d)), (V_(s) -V_(d)) and (V_(s) +V_(d)), c.f. FIG. 2, areshown along the V axis.

Referring once again to the complete addressing of the pixels 2 in agiven row, in step (a) the (erase) signal 12 of FIG. 2 is applied to thecorresponding row conductor 7 simultaneously with the application of thedata signals 8 and 9 to each of the column conductors. Each pixeltherefore has either the waveform 1 or the waveform 17 of FIG. 2 appliedthereacross. The former case corresponds to points 18, 19 and 18 in FIG.3 in succession, each of which lies within the region 14, and the lattercase corresponds to points 20, 21 and 20 in FIG. 3 in succession, point21 lying within the region 14. Thus all the pixels of the row which arenot already in a first state (determined by the polarity of thewaveforms 1 and 17 and in the present example an "off" state) are set tothat state. Subsequently in step (b) the (strobe) signal 10 of FIG. 2 isapplied to the corresponding row conductor 7 simultaneously with theapplication of one of the data signals 8 and 9 to each of the columnconductors. The data signal 8 is applied to those column conductorswhich correspond to pixels which it required remain in the first statewhereas the data signal 9 is applied to those conductors 6 whichcorrespond to pixels which it is required are finally set to a secondstate (in the present example an "on" state). Thus the former pixels aresupplied with the waveform 22 of FIG. 2 thereacross, whereas the latterpixels are supplied with the waveform 23 thereacross. Waveform 22corresponds to points 24,25 and 2 of FIG. 3 in succession, which pointsall lie within the region 15, and waveform 23 corresponds to points 24,27 and 28 of FIG. 3 in succession, point 27 lying within the region 14.Thus the pixels 2 which are supplied with the waveform 22 remain in thefirst or "off" state, whereas pixels which are supplied with thewaveform 23 are switched to the second or "on" state, the polarity ofwaveform 23 being opposite to the polarity of the waveforms 1 and 17.The overall result is therefore that selected pixels (those whose columnconductors are supplied with signal 8 in FIG. 2 in step (b) are set toor remain in the first or "off" state whereas other selected pixels(those whose column conductors are supplied with signal 9 in step (b))are set to or remain in the second or "on" state.

Steps (a) and (b) are performed for all the rows of pixels, therebysetting each pixel of the cell 1 to the "on" or "off" state as required.This may be done in several ways. Obviously step (a) may be performed inrespect of several rows simultaneously, if desired, whereas step (b) canbe performed in respect of only one row at any given time. Those rows inrespect of which neither step (a) nor step (b) is being performed in agiven data period 2T are supplied at the relevant time with signal 11 ofFIG. 2, i.e. with zero volts on the corresponding row conductors 7. Thissignal 11 combines with the data signals 8 or 9 at the relevant pixelsto produce the waveforms 30 or 31 respectively in FIG. 2, thesewaveforms both corresponding with points 24,29 and 24 in FIG. 3 insuccession. As both points 24 and 29 lie within the region 15 the statesof the relevant pixels remain unchanged at these times.

If step (a) is performed in respect of several, for example two, rowssimultaneously, it will be evident that each such step may occupy acorresponding number of adjacent data periods 2T, because the number ofdata periods basically required to perform this step in respect of allthe rows will be reduced by a corresponding factor.

It might be assumed from the above description that exactly equivalentresults would be obtained if the strobe signal 10 of FIG. 2 were toterminate three-quarters of the way through a data period 2T, ratherthan at the end thereof as described. This would mean that the finalquarters of the waveforms 22 and 23 would have amplitudes +V_(d) and-V_(d) respectively (corresponding to point 24 in FIG. 3) rather than-(V_(s) -V_(d)) and -(V_(s) +V_(d)) respectively (corresponding topoints 2 and 28 respectively in FIG. 3), all the points 24,26 and 28lying within the region 15. This is true if the absolute value of thedata period 2T is such as to position the point 27 well inside theregion 14 of FIG. 3 as indicated. However, if this absolute value isreduced in an attempt to reduce the time required to address all thepixels 2 of the cell 1 this will have the effect of moving all thepoints 18-21 and 24-29 towards the v axis. This is immaterial as far asthe points within the region 15 are concerned. However, those pointswithin the region 14 will move towards the boundary 13 between theregions 14 and 15, and eventually cross it if the data period 2T isreduced sufficiently. It has, however, been found that a greaterreduction in the absolute value of the data period 2T can beaccommodated without jeopardising the state switching effect of thenotional active (central) portion of waveform 23 of FIG. 2(corresponding to point 27 in FIG. 3) if this central portion issucceeded by a further portion of greater amplitude as shown(corresponding to point 28 in FIG. 3) rather than by a further portionof lesser amplitude (corresponding to point 24 in FIG. 3) as would beobtained if the strobe signal 10 of FIG. 2 were to terminatethree-quarters of the way through the relevant data period, even thoughboth the points 28 and 24 lie within the region 15 of FIG. 3. In otherwords, a greater reduction in the data period 2T is possible withoutjeopardising the state switching required in step (b) if the strobesignal terminates after the active or central portion of the data signal9 of FIG. 2 has ended rather than simultaneously with the end of thiscentral portion.

It will be appreciated that a reduction in the absolute value of eachdata period 2T also tends to jeopardise the state switching effect ofthe erase waveforms 16 and 17 of FIG. 2 because the points 18, 19 and 21of FIG. 3 will also move towards the boundary 13 between the regions 14and 15. However, this effect can be ameliorated by, as mentionedhereinbefore, performing step (a) in respect of more than one row ofpixels 2 simultaneously and arranging that each such step occupies aplurality of adjacent data periods 2T. This will mean that, when step(a) is performed in respect of any given pixel, that pixel will besupplied with one of the waveforms 16 and 17 of FIG. 2 immediatelyfollowed by at least one more of these waveforms so that the switchingeffect on the pixel will be given by a combination of two or more ofthese waveforms. Examination of the waveforms 16 and 17 will make itclear that such a combination will necessarily include a portioncorresponding to point 21 in FIG. 3, which point is furthest from theboundary 13 as described. In fact amelioration of the said effect can beobtained even if the data periods of the plurality do not follow eachother in direct succession, provided that they are reasonably close toeach other, for example within three data periods of each other.

It will be appreciated that many modifications may be made to theembodiment described, within the scope of the invention as defined bythe claims. For example, although preferred, the start of the strobesignal 10 of FIG. 2 need not necessarily coincide with the start of thesecond portion of the data signals 8 and 9; it may occur after thesesecond portions have commenced (but before the end of these secondportions) if desired. Moreover, the end of the strobe signal 10 need notnecessarily coincide with the end of the third portion of the datasignals 8 and 9 although, in accordance with the invention, it mustoccur after this third portion has commenced. Furthermore, the secondportions of the data signals 8 and 9 do not necessarily occupy exactlyhalf of a complete data period; they may, for example, occupythree-quarters of such a data period, the first and third portions theneach occupying one-eighth of a data period and having double theamplitude of the second portion so that the total area under the firstand third portions remains equal to the are under the second portion tomaintain balance. As yet another example the amplitudes of the strobeand erase signals 10 and 12 of FIG. 2 are not necessarily constantthroughout their duration; either or both may for example consist of asuccession of pulses which may or may not have the same amplitude.

Although as described the first and second states of each pixel are"off" and "on" states respectively, it will be evident that the reversemay be the case, if desired. Moreover the "off" and "on" states need notbe permanently stable; stability is required only for a time equal tothe maximum time elapsing between the application of successive eraseand strobe signals to the relevant pixel.

In one implementation of the method described with reference to FIGS.1-3 in which step (a) was carried out for two rows at a time andoccupied two adjacent data periods each time, various parameters etc.were as follows:

Liquid crystal material: Merck type 5014

Spacing between the electrodes 3 and 4 of each cell: 1.5 μm

V_(d) : 10 V

V_(s) : 30 V

V_(b) : 15 V

2T: 184 μs

Temperature: 35 ° C.

I claim:
 1. A method of addressing a matrix-type liquid crystal cellincluding liquid crystal material which is electrically settable tofirst and second stable optical states, the cell comprising a pluralityof pixels which are defined by areas of overlap between members of afirst set of electrodes on one side of the material and members of asecond set of electrodes, which cross the first set, on the other sideof the material, in which method the pixels are addressed in lines, theaddressing of each line comprising (a) applying an erase signal having agiven polarity to the corresponding electrode of the first set whileapplying at least one charge-balanced bipolar data signal to eachelectrode of the second set, thereby setting any pixel of the line whichis not already in the first state to that state, and (b) subsequentlyapplying a strobe signal having an opposite polarity to said givenpolarity to the corresponding electrode of the first set while applyinga charge-balanced bipolar data signal to each electrode of the secondset, thereby selectively setting to the second state any pixel of theline for which the data signal applied to the corresponding electrode ofthe second set has a given form, characterised in that when said datasignal has said given form it comprises first, second and thirdsuccessive portions in which it has said given polarity, said oppositepolarity and said given polarity respectively, the amplitude of thesecond portion being less than the amplitude of the strobe signal, theend of the first portion coinciding with or occurring before the startof the strobe signal, the end of the second portion occurring after thestart of the strobe signal, and the start of the third portion occurringbefore the end of the strobe signal.
 2. A method as claimed in claim 1,wherein the ends of the third portion and the strobe signal coincide. 3.A method as claimed in claim 2, wherein said first, second and thirdportions have equal amplitude, the first and third portions each beingof half the length of the second portion.
 4. A method as claimed inclaim 1, wherein said first, second and third portions have equalamplitude, the first and third portions each being of half the length ofthe second portion.
 5. A method as claimed in claim 1 wherein, instep(a), an erase signal having the given polarity is applied to thecorresponding electrode of the first set while applying a plurality ofcharge-balanced bipolar data signals to each electrode of the secondset.
 6. A method as claimed in claim 5, wherein said plurality ofcharge-balanced bipolar data signals follow each other in directsuccession.
 7. Optical modulator apparatus comprising a matrix-typeliquid crystal cell and an addressing signal generator for addressingsaid cell, the cell including liquid crystal material which iselectrically settable to first and second stable optical states, andcomprising a plurality of pixels which are defined by areas of overlapbetween members of a first set of electrodes on one side of the materialand members of a second set of electrodes, which cross the first set, onthe other side of the material, the addressing signal generator havingoutputs coupled to the first and second sets of electrodes and beingcontructed to address the pixels in lines by each time (a) applying anerase signal having a given polarity to the corresponding electrode ofthe first set while applying at least one charge-balanced bipolar datasignal to each electrode of the second set, thereby setting any pixel ofthe line which is not already in the first state to that state, and (b)subsequently applying a strobe signal having an opposite polarity tosaid given polarity to the corresponding electrode of the first setwhile applying a charge-balanced bipolar data signal to each electrodeof the second set, thereby selectively setting to the second state anypixel of the line for which the data signal applied to the correspondingelectrode of the second set has a given form, characterised in that thegenerator is constructed so that when said data signal has said givenform it comprises first, second and third successive portions in whichit has said given polarity, said opposite polarity and said givenpolarity respectively, the amplitude of the second portion being lessthan the amplitude of the strobe signal, the end of the first portioncoinciding with or occurring before the start of the strobe signal, theend of the second portion occurring after the start of the strobesignal, and the start of the third portion occurring before the end ofthe strobe signal.